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M. Rajeevan and J. Srinivasan

Abstract

Based on the data from Earth Radiation Budget Experiment (ERBE), many investigators have concluded that the net cloud radiative forcing at the top of the atmosphere is small in the deep convective region of the Tropics. This conclusion has been shown to be invalid for the Asian monsoon region during the period June–September. The ERBE data have been used to show that in the Asian monsoon region the net cloud radiative forcing at the top of the atmosphere is negative and its magnitude exceeds 30 W m−2 in 25% of the grids in this region. The large negative net cloud radiative forcing in the Asian monsoon region during June–September has been shown to be on account of the presence of large amount of high clouds and the large optical depth of these clouds. This combination of high cloud amount and high optical depth occurs in the Asian monsoon region only. In the other deep convective regions of the Tropics, high clouds with large optical depths are present, but they do not cover a large area.

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J. Srinivasan and G. L. Smith

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S. K. Satheesh and J. Srinivasan

Abstract

Radiative forcing of aerosols is much more difficult to estimate than that of well-mixed gases due to the large spatial variability of aerosols and the lack of an adequate database on their radiative properties. Estimation of aerosol radiative forcing generally requires knowledge of its chemical composition, which is sparse. Ground-based sky radiance measurements [e.g., aerosol robotic network (AERONET)] can provide key parameters such as the single-scattering albedo, but in shipborne experiments over the ocean it is difficult to make sky radiance measurements and hence these experiments cannot provide parameters such as the single-scattering albedo. However, aerosol spectral optical depth data (cruise based as well as satellite retrieved) are available quite extensively over the ocean. Spectral optical depth measurements have been available since the 1970s, and spectral turbidity measurements (carried out at meteorological departments all over the world) have been available for several decades, while long-term continuous chemical composition information is not available. A new method to differentiate between scattering and absorbing aerosols is proposed here. This can be used to derive simple aerosol models that are optically equivalent and can simulate the observed aerosol optical properties and radiative fluxes, from spectral optical depth measurements. Thus, aerosol single-scattering albedo and, hence, aerosol radiative forcing can be estimated. Note that the proposed method is to estimate clear-sky aerosol radiative forcing (over regions where chemical composition data or sky radiance data are not available) and not to infer its exact chemical composition. Using several independent datasets from field experiments, it is demonstrated that the proposed method can be used to estimate aerosol radiative forcing (from spectral optical depths) with an accuracy of ±2 W m−2.

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J. Srinivasan and G. L. Smith

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Outgoing longwave radiation data from the Earth Radiation Budget Experiment show that the meridional migration of tropical convergence zones (TCZ) varies greatly from one region to another in the Tropics. In Africa, the meridional migration of TCZ is limited due to the meridional variation of the planetary net radiation due to the Sahara Desert. The large meridional migration in the Asian subcontinent and the west Pacific Ocean is attributed to the strong land-sea contrast in these regions. The wind-evaporation feedback mechanism is proposed as the cause of intraseasonal meridional migration of TCZ in the Atlantic and east Pacific Oceans.

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J. Srinivasan and G. L. Smith

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The relationship between monthly mean outgoing longwave radiation (OLR) and the mean moist static energy of the lower troposphere is shown to be similar to the relationship between monthly mean OLR and sea surface temperature over the oceanic regions. The relationship between monthly mean OLR and surface moist static energy shows that the threshold value for the onset of convection is different in continental and oceanic regions. However, the threshold of moist static stability for the troposphere (surface to 400 mb) is the same for oceans and continents. This relationship is consistent with the simple model of the tropical convergence zones proposed by Neelin and Held. The net energy convergence in the troposphere was found to be positive in regions with OLR below 210 W m−2. This result is consistent with the Neelin and Held hypothesis that the necessary but not sufficient condition for the existence of tropical convergence zone is a positive energy convergence in the troposphere.

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S. K. Satheesh and J. Srinivasan
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Sulochana Gadgil, Asha Guruprasad, and J. Srinivasan

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The outgoing longwave radiation (OLR) fluxes derived from NOAA-SR (1974–78) are found to he consistently higher than those from NOAA-7 (1982 onward) over a large part of the tropical belt. Analysis of the variation of the mean July–August OLR and the rainfall over the Indian region suggests that the lower values of OLR in the latter period cannot be attributed to more intense convection. Thus, the consistently lower values of OLR in the latter period over a large part of the tropical belt (including the oceanic regions) may be a manifestation of a systematic bias arising from various factors such as changes in instruments, equatorial crossing time, etc. Obviously, if such a bias is present, it has to be removed before the dataset can be used for the study of interannual variations. If the bias is removed by a simple method based on the variation of convection over the entire tropical belt, the OLR variations over the Indian region become consistent with the rainfall variations.

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Satya Prakash, Ashwin Seshadri, J. Srinivasan, and D. S. Pai

Abstract

Rain gauges are considered the most accurate method to estimate rainfall and are used as the “ground truth” for a wide variety of applications. The spatial density of rain gauges varies substantially and hence influences the accuracy of gridded gauge-based rainfall products. The temporal changes in rain gauge density over a region introduce considerable biases in the historical trends in mean rainfall and its extremes. An estimate of uncertainty in gauge-based rainfall estimates associated with the nonuniform layout and placement pattern of the rain gauge network is vital for national decisions and policy planning in India, which considers a rather tight threshold of rainfall anomaly. This study examines uncertainty in the estimation of monthly mean monsoon rainfall due to variations in gauge density across India. Since not all rain gauges provide measurements perpetually, we consider the ensemble uncertainty in spatial average estimation owing to randomly leaving out rain gauges from the estimate. A recently developed theoretical model shows that the uncertainty in the spatially averaged rainfall is directly proportional to the spatial standard deviation and inversely proportional to the square root of the total number of available gauges. On this basis, a new parameter called the “averaging error factor” has been proposed that identifies the regions with large ensemble uncertainties. Comparison of the theoretical model with Monte Carlo simulations at a monthly time scale using rain gauge observations shows good agreement with each other at all-India and subregional scales. The uncertainty in monthly mean rainfall estimates due to omission of rain gauges is largest for northeast India (~4% uncertainty for omission of 10% gauges) and smallest for central India. Estimates of spatial average rainfall should always be accompanied by a measure of uncertainty, and this paper provides such a measure for gauge-based monthly rainfall estimates. This study can be further extended to determine the minimum number of rain gauges necessary for any given region to estimate rainfall at a certain level of uncertainty.

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Saroj K. Mishra, J. Srinivasan, and Ravi S. Nanjundiah

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Several numerical experiments have been conducted using the NCAR Community Atmosphere Model, version 3 (CAM3) to examine the impact of the time step on rainfall in the intertropical convergence zone (ITCZ) in an aquaplanet. When the model time step was increased from 5 to 60 min the rainfall in the ITCZ decreased substantially. The impact of the time step on the ITCZ rainfall was assessed for a fixed spatial resolution (T63 with L26) for the semi-Lagrangian dynamical core (SLD). The increase in ITCZ rainfall at higher temporal resolution was primarily a result of the increase in large-scale precipitation. This increase in rainfall was caused by the positive feedback between surface evaporation, latent heating, and surface wind speed. Similar results were obtained when the semi-Lagrangian dynamical core was replaced by the Eulerian dynamical core. When the surface evaporation was specified, changes in rainfall were largely insensitive to temporal resolution. The impact of temporal resolution on rainfall was more sensitive to the latitudinal gradient of SST than to the magnitude of SST.

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